1. Field of the Invention
This invention relates generally to semiconductor lasers, and more particularly to a semiconductor laser diode system with a control loop that provides a controlled output beam of selected wavelength in response to a changes in temperature of the diode or changes in output beam wavelength.
2. Description of Related Art
There are stringent requirements on the stability of the output wavelength and the output power of semiconductor laser units used for optical communication, and specifically for dense wave division multiplexing (DWDM). Typically, the lasers used for these applications are distributed feedback (DFB) semiconductor lasers. At fixed driving current, the output power and wavelength of these devices varies strongly with temperature. Typically the wavelength change is 0.6 xc3x85/xc2x0 C. Over an operating range of 0 to 75 degrees Celsius, this wavelength change is 4.5 nm. In DWDM systems the channel spacing is 0.8 nm or 0.4 nm so this wavelength variation is unacceptable. In addition, aging of the laser will also cause a change in wavelength. To overcome this problem DFB lasers are packaged with a temperature sensor and a cooler which operate in a closed loop to maintain the laser at a fixed temperature. At a fixed temperature the laser wavelength variation is minimized. Thermal control systems used with laser diodes have included the combination of a photodiode, thermoelectric cooler and thermistor. The photodiode receives a portion of the output of the diode laser. The output from the laser diode is stabilized by a feedback drive circuit and a reference adjust variable resistor. A temperature feedback circuit uses a thermistor and the reference adjust resistor to provide feedback stabilization of the laser diode temperature.
U.S. Pat. No. 5,602,860 discloses a cooling system for a laser diode that includes a temperature sensitive switch. When a temperature exceeds a given temperature the switch is open. Coupled to the switch is a thermoelectric cooler that cools the laser diode.
To provide more precise control of the wavelength, the wavelength can be monitored. U.S. Pat. No. 5,867,513 discloses a semiconductor laser unit in which the temperature is controlled in a closed loop which monitors the output wavelength of the laser.
Thermoelectric coolers generally require larger, more expensive power supplies than would otherwise be used. The use of thermoelectric coolers has proven to waste a significant amount of power from the power source. In one device, one or more voltage regulators are used to regulate the power supplied to a thermoelectric cooler. The power used by the voltage regulator is wasted. In addition the thermoelectric cooler itself adds to the cost of manufacturing the laser. U.S. Pat. No. 5,387,974 discloses one embodiment of a temperature insensitive wavelength meter and wavelength compensation for a KrF excimer laser as a method of avoiding the use of costly temperature control. Excimer lasers produce light in the UV wavelength range and are not appropriate for communications applications.
There is a need for a passively cooled wavelength stabilized laser system suitable for use in WDM communication applications and systems.
Accordingly, an object of the present invention is to provide a laser system.
Another object of the present invention is to provide a wavelength stabilized laser system.
A further object of the present invention is to provide a passively cooled wavelength stabilized laser system.
Yet another object of the present invention is to provide a passively cooled wavelength stabilized laser system useful in WDM communications.
These and other objects of the invention are achieved in a tunable semiconductor laser system. Included is a laser with a semiconductor active region positioned between upper and lower confining regions of opposite type semiconductor material. First and second reflective members are positioned at opposing edges of the active and confining regions. A wavelength tuning member and a temperature sensor are coupled to the laser. A control loop is coupled to the temperature sensor and the tuning member. In response to a detected change in temperature the control loop sends an adjustment signal to the tuning member and the tuning member adjusts a voltage or current supplied to the laser to provide a controlled output beam of selected wavelength.
In another embodiment, a tunable semiconductor laser system includes a laser with an electrically responsive substrate. A support block is positioned on the electrically responsive substrate. Also included are top and bottom reflectors. A first cantilever structure includes a base section that rests on the support block. A deformable section extends above the electrically responsive substrate, creating an air gap between the deformable section and the electrically responsive substrate. An active head is positioned at a predetermined location on the deformable section and includes at least a portion of the top reflecting member. A wavelength measurement member is positioned to receive at least a portion of the output beam of the laser. A control loop is coupled to the a wavelength measurement and the tuning member. In response to a detected change in wavelength, the control loop sends an adjustment signal to the tuning member. The tuning member adjusts a voltage or current supplied to the laser to provide a controlled output beam of selected wavelength.